What a difference a decade makes: progress in ab initio studies of the hydrogen bond

This article provides a summary of our studies of hydrogen-bonded complexes during the decade of the 90's. These studies began with systematic investi gations of the methodological dependence of the computed structures and bin ding energies of these complexes. The MP2/6-31 + G(d,p) level of theory was identified as the minimum level required to obtain reliable structures, wh ile reliable energetics required larger polarized split-valence basis sets that include diffuse functions. While the experimental frequency shift of t he A-H stretching band upon formation of an A-H-B hydrogen bond could also be reproduced at MP2/6-31+G(d,p) for a variety of hydrogen-bonded complexes , significant discrepancies were observed for others, including complexes o f HCl and HBr with ammonia, trimethylamine, and 4-substituted pyridines. Re solving these discrepancies became the primary focus of our work, and redef ined our research efforts. We solved a model two-dimensional nuclear Schrod inger equation to obtain anharmonic dimer- and proton-stretching frequencie s, modeled matrix effects with external electric fields, and characterized hydrogen bond types as traditional, proton-shared, and ion-pair. We were ab le to resolve the observed discrepancies between theory and experiment, and explain the rather disparate effects of matrices on the IR spectra of clos ely related complexes. We also initiated studies of the NMR properties of t he chemical shift of the hydrogen-bonded proton, and the A-B spin-spin coup ling constant across the A-H-B hydrogen bond. We demonstrated the dominance of the Fern-ii-contact term for determining coupling constants in complexe s with N-H-N, N-H-O, O-H-O, and Cl-H-N hydrogen bonds, and the distance dep endence of this term. We also showed that the IR anharmonic proton-stretchi ng frequency and the NMR spin-spin coupling constant are spectroscopic fing erprints of hydrogen bond type, which provide information about intermolecu lar distances in hydrogen-bonded complexes. (C) 2001 Elsevier Science B.V. All rights reserved.